The present invention relates to an antenna device for use with a mobile communication terminal, and more particularly, to an antenna device for use with a cellular phone.
Cellular phones are used in various countries. As shown in FIGS. 8A through 8D, an antenna is classified into several different types, which vary from country to country. FIG. 8A shows a whip antenna of top loading type in the extended state and in the housed state. FIG. 8B shows an antenna of bottom loading type in the extended state and in the housed state. The cellular phones shown in FIGS. 8A and 8B are of retractable type. FIG. 8C shows an antenna of externally fixed type, and FIG. 8D shows an antenna of built-in type.
An antenna which is a combination of a retractable-type antenna and a built-in-type antenna has recently been proposed. The reason for employment of such a hybrid type is that a deficiency arises in the sensitivity of an antenna when either one of the external fixed type or the built-in type is adopted, which in turn interferes with communication. An additional reason is that, in terms of design, demand exists for an antenna structure which eliminates projection which would otherwise arise when an antenna is stored.
Satisfying both requirements requires a structure which connects a feeding circuit to a whip or built-in antenna by way of a changeover switch.
However, no switch suitable for use as such a changeover switch has hitherto been available. Changeover switches are classified into an electrical switch using a pin diode, and a mechanical switch. When an electric current passes through a changeover switch of former type, a large loss arises. A changeover switch of latter type includes a high-frequency switch which involves a lower passage loss but is bulky and expensive. Thus, both types of changeover switches involve problems.
The present invention has been conceived in consideration of the foregoing point and aims at providing an antenna for a compact mobile communication terminal which enables mechanical switching between a whip antenna and a built-in antenna and involves lower signal loss.
In order to achieve the object, according to the present invention, there is provided an antenna device for a mobile communication terminal, comprising:
a whip antenna, which is movable between a first position corresponding to a state in which the whip antenna is extended from a casing of the mobile communication terminal, and a second position corresponding to a state the whip antenna is accommodated within the casing;
a built-in antenna, disposed within the casing;
a power feeding circuit, disposed within the casing;
a first matching circuit having a first impedance, through which the power feeding circuit and the whip antenna are electrically connected when the whip antenna is placed at the first position;
a second matching circuit having a second impedance which is higher than the first impedance, through which the whip antenna is grounded when the whip antenna is placed at the second position; and
a switch mechanism, which is opened so that the built-in antenna is electrically disconnected from the power feeding circuit when the whip antenna is placed at the first position, and closed so that the built-in antenna is electrically connected to the power feeding circuit.